Stereoselectivity of Metathesis of Various Acyclic Olefins with Various Group VI Transition Metal Catalysts

Summary

With the catalyst Mo (NO) ₂X₂ (PPh₃)₂ + EtAlCl₂, X=Cl, Br, I, the stereoselectivity of metathesis of cis and trans R−CH = CH−CH₃ has been determined by the trans/cis ratio of but-2-enes (C₄) at 0% conversion. For R=H, trans/cis C₄ is equal to 1. For R=Et, trans/cis ratios of C₄ are equal to 0.20 and 12.5 respectively for cis and trans olefin; increasing bulkiness of R results is a significant decrease of stereoselectivity (and activity): with a cis olefin the trans/cis ratio of C₄ increases when the bulkinesse of R increases (trans/cis ratio = 0.44 for R=t-C₄H₉). With a trans olefin the trans/cis ratio of but-2-ene decreases when the R size increases (trans/cis ratio of but-2-ene = 8.3 for R=t−C₄H₉). This effect does not seem to be the result of an electronic effect of the R group since similar stereoselectivities are obtained with trans R−CH=CH−CH₃ where R = t−Bu, phenyl or totyl. Changing the coordination sphere of the precursor catalyst (X = Cl, Br, I) does not result in a meaningful variation of stereoselectivity whereas going from Cr to Mo to W does decrease the stereoselectivity. With cis or trans pent-2-ene, the trans/cis ration of hex-3-enes is always higher than the trans/cis ratio of but-2-enes, with M = Mo or W. This result which is related to a 1–2 repulsive a-e ⇄ e-a interaction in the metallocyclobutane is corroborated by the stereoselectivities of the metathesis of α olefins : propylene, but-1-ene and pent-1-ene. The trans/cis ratio of the products increases in the order : but-2-ene (1.1.) < hex-3-ene (1.42) < oct-4-ene (2.0). If one assumes that stereoselectivity reflects steric interaction during olefin coordination or in the metallo-cyclobutane intermediates, simultaneous steric repulsions seem to account for the observed selectivities : I-2, I-3 and olefin-metal (and or its ligands).

Geometric isomerization of internal acyclic olefins was observed in the early works related to the metathesis reaction (1). Various preliminary studies seem to indicate that this geometric isomerization proceeds according to a metathetic mechanism (1,6). We wish concerning the stereochemistry of this reaction, data which proved to be helpful in the understanding of the reaction mechanism (7). In fact, if one considers recent data on the metathesis reaction (7 – 14), it appears that this reaction can be divided in three distinct reaction paths which can be refered to as :

• (1) Formal methathesis (1) ;

• (2) Degenerate metathesis (12, 15, 16) ;

• (3) cis-trans isomerization (1, 2, 4, 6, 7).

For α olefins only reactions (1) and (2) occur. For internal olefins these three reactions seem to occur to a great extent simultaneously ; it appears therefore that a stereochemical approach is quite helpful in elucidating the mechanism for the following reason : if cis-trans isomerization or degenerate metathesis are probable alternatives to metathesis, there must be a simple kinetic reatlion between the respective rates of reactions (1), (2) and (3) and tbe various probabilities of olefin coordination (and (or) reaction) to the active catalyst (metallo-carbene fragment). Since reaction (2) is quite difficult to study at very low conversion with labelled olefins we have focussed our research on the kinetic studies related to reactions (1) and (3). Besides, in a more detailed approach, we have studied the stereoselectivity of the olefin metathesis reaction as a function of (i) the cis or trans nature of the starting olefin, (ii) the nature of substituants on the starting olefin, (iii) the nature of the transition metal (group VI) in the precursor catalysts, and (iv) the nature of the ligands of the transition metal.